In response to the preovulatory LH surge, the preovulatory follicle rapidly increases progesterone production, which is essential for ovulation and/or corpus luteum formation. Although this rise in progesterone level has been mainly linked to a few LH-induced genes involved in steroidogenesis (e.g., StAR, Cypi 1 a l , and HSD3b), but there are many gaps in our understanding of periovulatory accumulation of progesterone. In this proposal, we put forward a novel protein, CIPAR1 (castration-induced prostatic apoptosis-related protein 1), as a vital mediator of progesterone accumulation in the periovulatory follicle. We have recently demonstrated the LH surge increases the expression of Ciparl in periovulatory follicles of rodent ovaries. More importantly, our preliminary study using granulosa cell cultures showed that knockdown of Ciparl expression resulted in significant reduction of progesterone levels. Based on these novel findings, we hypothesize that induction of CIPAR1 by the LH surge is crucial for progesterone accumulation in periovulatory follicles, thus ovulation and luteinization. In specific Aim#1, we will demonstrate that the alteration of Ciparl expression by silencing or over-expression affects progesterone production in rat periovulatory follicles using both in vivo and in vitro models. As the first approach to pinpoint the functional contribution of C1PAR1 in periovulatory follicles, we will identify the genes and proteins that are differentially expressed when Ciparl expression is silenced or over-expressed in periovulatory follicles. Because littie is known about cellular function of CIPAR1, Specific Aim #2 focuses on first determining the cellular location of CIPAR1 in periovulatory granulosa cells using confocal co-localization and/or subcellular fractionated protein analyses and then identifying CIPAR1 interacting proteins in periovulatory granulosa cells using immunoprecipitation, followed by a proteomic approach. Other than our preliminary data detecting Ciparl expression in human granulosa cells, nothing is known about CIPAR1 in human tissues. In collaboration with Drs. Brannstrum and Duffy, specific Aim #3 will determine whether Ciparl expression is hormonally regulated during the periovulatory period and critical for LH-induced progesterone production in humans and/or macaques. Data obtain from the proposed studies will unravel the role of C1PAR1 as a novel and critical mediator of progesterone accumulation in periovulatory follicles of primates as well as rodents.